Variable gain amplifier

ABSTRACT

A method may include applying an input differential voltage to input terminals of an amplifier, a first input terminal coupled to a gate of a first transistor and a second input terminal coupled to a gate of a second transistor. The method may also include varying a gain of the amplifier by varying at least one of: a resistance of a first resistor, the first resistor coupled between a source of the first transistor and a source of the second transistor; and a resistance of a second resistor, the second resistor coupled between a source of a third transistor and a source of a fourth transistor; wherein: the third transistor is coupled at its drain to the drain of the first transistor; and the fourth transistor is coupled at its drain to the drain of the second transistor and a gate of the third transistor and coupled at its gate to the drain of the third transistor.

RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.13/477,926 filed May 22, 2012, which is incorporated herein by referencein its entirety.

TECHNICAL FIELD

The present invention relates generally to electrical circuits, and moreparticularly, to variable gain amplifiers.

BACKGROUND

An electronic amplifier is a device for increasing the power of asignal. In basic operation, an electronic amplifier uses energy from apower supply and controls its output to match an input signal shape butwith a larger amplitude. Amplifiers are used in many electronicsapplications, including in communications, audio systems, servo motorcontrollers, and many other applications.

A key operational parameter of any amplifier is its gain. A gain istypically a multiplicative factor relating the magnitude of the outputsignal to the input signal. The gain may be specified as the ratio ofoutput voltage to input voltage (voltage gain), output power to inputpower (power gain), or some combination of current, voltage and power.In some applications, it is beneficial that an amplifier have a variablegain, wherein the gain of the amplifier may be modified or changed.However, existing approaches to adapting conventional amplifiers toprovide for variable gain have disadvantages.

To illustrate, reference is made to FIG. 1, which depicts a conventionalamplifier 100, as is known in the art. A conventional amplifier maycomprise transistors 108 a, 108 b, drain resistors 112 a, 112 b, andcurrent source 106. Each drain resistor 112 a, 112 b may be coupledbetween the drain terminal of a corresponding transistor 108 a, 108 b,respectively, and a high potential voltage source (e.g., V_(DD)). Eachtransistor 108 a, 108 b may in turn be coupled at its gate terminal to adifferential input (e.g., V_(in) ⁺ or V_(in) ⁻) and at its sourceterminal to current source 106.

In operation, transistors 108 a, 108 b form a differential transistorpair to sense and track the input data represented by a differentialinput voltage V_(in), and producing a differential output voltageV_(out) at the drain terminals of transistors 108 a, 108 b. Those ofskill in the art will appreciate that the gain A of amplifier 100 may begiven by the equation A=g_(m1)·R_(D), where g_(m1) is thetransconductance of each transistor 108 a, 108 b and R_(D) is theresistance of each resistor 112 a, 112 b. Thus, to make the gain Avariable, one or both of g_(m1) and R_(D) must be made variable. Forexample, R_(D) may be variable if resistors 112 a, 112 b are replacedwith variable resistors. As another example, those of skill in the artwill appreciate that g_(m1) is a function of I_(SS), the tail currentgenerated by current source 106. Accordingly, g_(m1) may be varied byvarying I_(SS), which it turn varies the gain A. However, as those ofskill in the art will appreciate, the common mode output voltageV_(cm, out) of amplifier 100 may be given by the equationV_(cm, out)=V_(DD)−I_(SS)·R_(D)/2. Thus, adjusting the gain by changingeither of g_(m1) or R_(D) will lead to a change in the output commonmode voltage. Changing the output common mode of amplifier 100 may beundesirable, as it disturbs the bias condition of the circuit.

SUMMARY

In accordance with some embodiments of the present disclosure, a methodmay include applying an input differential voltage to input terminals ofan amplifier, the input terminals comprising a first input terminal anda second input terminal, the first input terminal coupled to a gateterminal of a first transistor and the second input terminal coupled toa gate terminal of a second transistor. The method may also includevarying a gain of the amplifier by varying at least one of: a resistanceof a first resistor, the first resistor coupled between a sourceterminal of the first transistor and a source terminal of the secondtransistor; and a resistance of a second resistor, the second resistorcoupled between a source terminal of a third transistor and a sourceterminal of a fourth transistor; wherein: the third transistor iscoupled at its drain terminal to the drain terminal of the firsttransistor; and the fourth transistor is coupled at its drain terminalto the drain terminal of the second transistor and a gate terminal ofthe third transistor and coupled at its gate terminal to the drainterminal of the third transistor.

One or more technical advantages of the present disclosure may bereadily apparent to one skilled in the art from the figures,descriptions, and claims included herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 illustrates a schematic diagram of an example conventionalamplifier, as is known in the art; and

FIG. 2 illustrates a schematic diagram of a variable gain amplifier, inaccordance with certain embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure may refer to the “size” of various types oftransistors, including an n-type metal-oxide semiconductor field-effecttransistor (NMOS). Unless otherwise specified, the description of atransistor's size, as used herein, describes the size parameter thataffects the transconductance of the transistor. For example, for NMOSdevices, “size” may refer to the width-to-length ratio of the gateand/or of the conducting channel of the device. Accordingly, a devicethat is described as being sized at a ratio as compared to anotherdevice may have a transconductance that is larger or smaller at thatratio as compared to the transconductance of the other device.

Further, the term “equivalent” may be used to describe two or morecurrents or two or more voltage potentials that may be designed to beapproximately equal to each other. Though they may be designed to beapproximately equal to each other, “equivalent” voltages, “equivalent”currents, or other “equivalent” items may include some variation due tofactors including, but not limited to, device matching imperfections,semiconductor processing imperfections, and/or imbalanced operatingconditions.

FIG. 2 illustrates a schematic diagram of an example variable gainamplifier, in accordance with certain embodiments of the presentdisclosure. Power may be supplied to variable gain amplifier 200 througha high potential power supply and a low potential power supply. For thepurposes of this disclosure, a high potential power supply may bereferred to as “V_(DD)”, and a low potential power supply may bereferred to as “GND” or “V_(SS)”.

As shown in FIG. 2, variable gain amplifier 200 may comprise a firststage 202 and a second stage 204. First stage 202 may comprisetransistors 208 a, 208 b, resistors 212 a, 212 b, variable resistor 216,and current sources 206 a, 206 b. Each resistor 212 a, 212 b may becoupled between the drain terminal of a corresponding transistor 208 a,208 b, respectively, and a high potential voltage source (e.g., V_(DD)).Transistors 208 a, 208 b may in turn be respectively coupled at theirgate terminals to opposite polarities of a differential input (e.g.,V_(in) ⁺ or V_(in) ⁻). In addition, each transistor 208 a, 208 b may becoupled at its source terminal to a corresponding current source 206 a,206 b, respectively. In some embodiments, transistors 208 a and 208 bmay be of approximately the same size. Furthermore, variable resistor216 may be coupled between the source terminals of transistors 208 a and208 b.

Second stage 204 may comprise transistors 214 a, 214 b, variableresistor 218, and current sources 207 a, 207 b. Each of transistors 214a, 214 b may be coupled at its drain terminal to the drain terminal of acorresponding first stage transistor 208 a, 208 b, respectively.Furthermore, each transistor 214 a, 214 b may be coupled at its sourceterminal to a corresponding current source 207 a, 207 b, respectively.In addition, transistors 214 a and 214 b may be cross-coupled such thatthe gate terminal of transistor 214 a is coupled to the drain terminalof transistor 214 b, and vice versa. In some embodiments, transistors214 a and 214 b may be of approximately the same size. Variable resistor218 may be coupled between the source terminals of transistors 214 a and214 b.

A gain A of amplifier 200 may be given by A=G_(m1·R) _(L), where G_(m1)is the effective transconductance of first stage 202 as seen as seen atthe output V_(out) of amplifier 200, and R_(L) is the effective loadresistance as seen at the output V_(out) of amplifier 200. The effectivetransconductance G_(m1) may be given by:G _(m1) =g _(m1)/(1+g _(m1) ·R _(S1)/2)where g_(m1) is the transconductance of each transistor 208 a and 208 band R_(S1) is the resistance of variable resistor 216.

The effective load resistance R_(L) may be given by:R _(L) =R _(D)/(1−G _(m2) ·R _(D))where R_(D) is the resistance of resistors 212 a, 212 b and G_(m2) isthe effective transconductance of second stage 204 as seen at the outputV_(out) of amplifier 200. The transconductance G_(m2) may be given by:G _(m2) =g _(m2)/(1+g _(m2) ·R _(S2)/2)where g_(m2) is the transconductance of each transistor 214 a and 214 band R_(S2) is the resistance of variable resistor 218.

By variable substitution, the gain A may be given by:

$A = {\frac{g_{m\; 1}}{1 + {g_{m\; 1} \cdot \frac{R_{S\; 1}}{2}}} \cdot \frac{R_{D}}{1 - {\frac{g_{m\; 2}}{1 + {g_{m\; 2} \cdot \frac{R_{S\; 2}}{2}}} \cdot R_{D}}}}$

Accordingly, as seen by the above equations, negative resistance isadded by second stage 204 in order to modulate the effective loadresistance R_(L). In addition, so long as transistors 214 a, 214 b(corresponding to g_(m2)) and the resistance R_(S2) of variable resistor218 are selected such that effective load resistance R_(L)>0, amplifier200 will operate as an amplifier. Variable gain of amplifier 200 may beaccomplished by modifying one or both of variable resistors 216 and 218,without affecting the common mode voltage of the output of amplifier 200(which may be given by the equation V_(cm, out)=V_(DD)−I_(SS)·R_(D)/2,where V_(DD) is the supply voltage, I_(SS) is the tail current generatedby each of current source 206 a, 206 b, and R_(D) is the resistance ofeach of resistor 212 a, 212 b).

Although FIG. 2 depicts current sources 206 a, 206 b, 207 a, and 207 ball generating tail currents equal to I_(SS), other embodiments in whichcurrents generated by current sources 206 a and 206 b may besubstantially different than those generated by current sources 207 aand 207 b.

As used herein, a current source (e.g., current source 206 a, 206 b, 207a, and/or 207 b) may include any electrical or electronic deviceconfigured to deliver or absorb electric current.

As used herein, a transistor (e.g., transistor 208 a, 208 b, 214 a,and/or 214 b) may comprise any system, device, or apparatus configuredto control a signal at a pair of its terminals by a signal applied toanother pair of its terminals. Although the FIGURES of this disclosuredepicts transistors as n-type complementary metal-oxide-semiconductorfield-effect transistors, transistors may include any other suitabletype of transistor (e.g., n-type complementary metal-oxide-semiconductorfield-effect transistors, bipolar junction transistors, junction-gatefield effect transistors, insulated gate bipolar transistors, etc.).

As used herein, a resistor (e.g., resistor 212 a and/or 212 b) maycomprise any electronic component that produces a voltage across itsterminals that is proportional to the electric current passing throughit in accordance with Ohm's law.

As used herein, a variable resistor (e.g., variable resistor 216 and/or218) may comprise any electronic component that produces a voltageacross its terminals that is proportional to the electric currentpassing through it in accordance with Ohm's law, wherein such proportionbetween voltage and current may be varied based on a control signal orother stimulus provided to the variable resistor.

All examples and conditional language recited herein are intended forpedagogical objects to aid the reader in understanding the invention andthe concepts contributed by the inventor to furthering the art, and areto be construed as being without limitation to such specifically recitedexamples and conditions, nor does the organization of such examples inthe specification relate to a showing of the superiority and inferiorityof the invention. Although the embodiment(s) of the present inventionshave been described in detail, it should be understood that the variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A method, comprising: applying an inputdifferential voltage to input terminals of a variable gain amplifier,the variable gain amplifier comprising a gain stage and an active loadstage; and varying a gain of the variable gain amplifier withoutaltering a common mode output voltage of the variable gain amplifier byvarying an effective impedance provided by the active load stage to thegain stage, wherein varying the effective impedance includes sourcedegeneration of the active load stage.
 2. The method of claim 1, whereinvarying the gain includes source degeneration of the gain stage.
 3. Anamplifier, comprising: a gain stage; and an active load stagecomprising: a first transistor directly coupled at its drain terminal tothe gain stage and to a gate terminal of a second transistor; the secondtransistor directly coupled at its drain terminal to the gain stage andto a gate terminal of the first transistor; a first current sourcecoupled to a source terminal of the first transistor; a second currentsource coupled to a source terminal of the second transistor; and avariable resistor coupled between the source terminal of the firsttransistor and the source terminal of the second transistor, wherein again of the amplifier is modified without altering a common mode outputvoltage of the amplifier by: varying an effective impedance provided bythe active load stage to the gain stage.
 4. The amplifier of claim 3,wherein varying the effective impedance includes source degeneration ofthe active load stage.
 5. The amplifier of claim 4, wherein the gain ofthe amplifier is modified by source degeneration of the gain stage. 6.The amplifier of claim 4, wherein the source degeneration of the activeload stage includes varying a resistance of the variable resistor.